Fuel cell stacks typically comprise a plurality of fuel cells stacked one upon the other and held in compression with respect to each other. The plurality of stacked fuel cells form a fuel cell assembly which is compressed to hold the plurality of fuel cells in a compressive relation. Typically, each fuel cell comprises an anode layer, a cathode layer, and an electrolyte interposed between the anode layer and the cathode layer. The fuel cell assembly requires a significant amount of compressive force to squeeze the fuel cells of the stack together. The need for the compressive force comes about from the internal gas pressure of the reactants within the fuel cells plus the need to maintain good electrical contact between the internal components of the cells. Generally, the per area unit force is about 195-205 psi total which is distributed evenly over the entire active area of the cell (typically 77-155 square inches for automotive size stacks). Thus, for a fuel cell with an area of about 80 square inches, the typical total compressive force of these size stacks is about 15,500 to 16,500 pounds.
Conventional fuel cell stack structures focused on the use of rigid end plates and tie rods to apply and maintain a compressive force on the fuel cell assembly. The plurality of fuel cells or fuel cell assembly to be compressed is interposed between a pair of rigid end plates. The end plates are then compressed together by tie rods that extend through or around the end plates and impart a compressive force on the end plates. Additionally, the tie rods typically extend beyond the surface of the end plates and thereby increase the volume of the stack structure. When the stack structure utilizes tie rods distributed around a periphery of the end plate to impart a compressive force on the fuel cell assembly, the proper tightening of the tie rods to impart the desired compressive force can be difficult. That is, the tie rods are tightened in a predetermined pattern in an attempt to apply in an evenly distributed compressive load on the fuel cell assembly. However, as each tie rod is tightened the compressive load being imparted by the end plates changes so that each tie rod must be re-tightened multiple times in an iterative process in order to achieve a generally uniform compressive force on the fuel cell assembly. Additionally, the tie rods typically extend beyond the surface of the end plates and thereby increase the volume of the stack structure.
Typical applications in which fuel cells are used require the fuel cell assembly to be enclosed in a protective casing. The typical protective casing is applied over the existing stack structure and adds volume to the overall stack structure. The protective casing thereby increases the size of the stack structure with no utility being gained from the increased size other than the protection afforded thereby. Because the fuel cells are typically used in applications where space is a premium, it is desirable to provide a fuel cell that is contained within a protective casing that is of a minimal volume.
Therefore, it would be advantageous to provide a stack structure that can more easily impart a compressive force on the fuel cell assembly, and even more advantageous if the compressive force applying means added minimal volume to the stack structure. Furthermore, it would be advantageous to provide a protective casing for a fuel cell assembly that adds a minimal volume to the stack structure, and even more advantageous if the protective enclosure provides benefits to the stack structure in addition to the protection of the fuel cell assembly.